1. Field of the Invention
The present invention relates to a motor controller and an electric power steering apparatus provided with the motor controller.
2. Description of the Prior Art
It is well known for a motor controller of an electric power steering apparatus (EPS) to stop a controller controlling its motor immediately after abnormal state is detected. The abnormal state is a current-carrying failure in any phase of U, V, W of the motor by a wire breaking of a power supplying line and a terminal failure of a driving circuit. Therefore, the motor controller is provided with an abnormal detecting member to detect a generation of the current-carrying failure and to stop the motor control for a fail safe immediately after the detection of the abnormal generation.
However, there is a problem for the conventional EPS to change extremely its steering performance by the stop of the motor control. Therefore, a driver of a vehicle is required larger steering force for a steady steering operation. In this point, there is a conventional motor controller as disclosed in a Japanese unexamined published patent document 2003-26020 (Tokkai 2003-26020) to continue the motor control by carrying the current into other remaining two phases than the phase of the current-carrying failure. Thereby, the conventional motor controller can keep exerting an assist force to a steering system to prevent increasing of a load to the driver by the fail safe.
However, the conventional motor controller tends to make a worse steering feeling due to a generation of a torque ripple because of the construction to carry the current like a sine wave in a V phase and a W phase (the current-carrying failure in a U phase) of the current-carrying phases as shown in FIG. 1.
In the conventional motor controller as shown in FIG. 2, where a transition of the motor current at the two phases driving is shown by a d/q coordinate system, even though a q-axis current command value as a target control value of the motor torque is constant, an actual q-axis current value, however, varies like the sine curve. As a result, the motor current corresponding to a required torque does not appear so that the motor driving is continued without its original performance.